Blue Moon in 2021: When Is the Next Blue Moon?

Catch the Sturgeon Blue Moon This August!

August 5, 2021
Blue Moon - OFA

The Sturgeon Blue Moon will occur on August 21-22, 2021! What is a Blue Moon, exactly? Read all about Blue Moons.

Catch the Sturgeon Blue Moon

August’s full Moon, which is traditionally known as the Sturgeon Moon, is extra-special this year: it’s a seasonal Blue Moon!

Most of the time, a season (winter, spring, summer, or fall) contains three full Moons. However, if the dates of the full Moons and the seasons line up just right, a season may end up containing four full Moons instead. If that happens, the third full Moon of the season is called a seasonal Blue Moon! That’s exactly the case in August 2021.

When to See the Sturgeon Blue Moon

This full Moon will appear on the night of Saturday, August 21, before reaching peak “fullness” the next morning (when it will be hidden below the horizon). Venture outside just after sunset and keep an eye on it through the night to see it swim slowly across the sky!

→ Why is August’s Moon called the Sturgeon Moon? Learn all about it on our August Full Moon page!

What Is a Blue Moon?

Need a refresher on what a Blue Moon actually is? We get a lot of questions about the term, including:

  • Is “Blue Moon” a scientific term used in astronomy?
  • Did this term originate with Native American folklore, like a number of the other full Moon names?
  • Does a Blue Moon really look blue?

In truth, the answer to all of these questions is “no.”

The modern understanding of “Blue Moon” only took off in the 1980s. It was a result of a much earlier mistake printed in a 1946 issue of Sky & Telescope magazine, and since then, the term has gone viral in the media.  

Two Types of Blue Moons

There are two definitions of the term that are commonly used today:

  • Seasonal Blue Moon: The extra full Moon that occurs within an astronomical season. One season—defined by the dates of the solstices and equinoxes—typically has three full moons occur within it. If a season instead has four full moons, then the third full moon (not the fourth) in the season may be called a Blue Moon. 
  • Calendrical Blue Moon: The second full moon to occur in a calendar month. It takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon), so if a full Moon occurs on the first of a month, there will be a second full Moon—a Blue Moon—at the end of the month, too (except in February).

Although the latter definition is the one more commonly followed today, the former actually came first. As mentioned above, a misinterpretation of the seasonal definition in the 1940s gave way to the calendrical definition, which was later popularized in the 80s and has stuck around to today.


When Is the Next Blue Moon?

As of this writing, the next Blue Moon according to either definition will occur on August 22, 2021, as described in detail above.

After that, we’ll have to wait a couple of years for another Blue Moon:

  • The next calendrical Blue Moon will happen on August 30, 2023, as the second full Moon in that month.

How Often Does a Blue Moon Occur?

Most months have one full Moon, not two.

Since the Moon’s period of phases is 29 ½ days, while months usually have 30 or 31 days, it’s obvious that if a full Moon lands on the first day of any month except February, it will repeat again at the end. 

Turns out, calendrical Blue Moons happen every 30 months on average. Two and a half years. Seasonal Blue Moons happen at a similar rate: about once every two to three years. So maybe “once in a Blue Moon” isn’t so rare after all! 

Two Blue Moons in a Single Year

But how often do we have two Blue Moons in a single year? (As in 2018, when there were Blue Moons in both January and March, and no full Moon in February.)

This won’t happen again until 2037, when we’ll again have another Blue Moon in both January and March.

Learn More

Want to learn more about the Moon? See the Almanac’s Moon Phase Calendar and Full Moon Calendar!

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe